The Text Telephone Type (TTY)/Text telephone Devices for the Deaf (TDD) service provides mobile text voice communication for the deaf. Currently, TTY standards include V.21, EDT, and Baudot standards.
In the process of implementing a TTY service, text information (in the T.140 format) of a user is encoded by using the Baudot standard and sent to a network. The inventor finds that after the Nb/Nc interface between one Mobile Switching Center (MSC) and another MSC is IP-based, a Baudot signal needs to be transmitted in the IP network if a transmission method in the prior art is applied. However, because the Internet Protocol (IP) network generally employs a compressed voice codec format such as Adaptive Multi-Rate (AMR) which provides a very high compression ratio, the Baudot signal is impaired drastically, so that it is hard to recover the Baudot signal correctly on the called side.